1. Technical Field
The present invention generally relates to ceramic materials. More particularly, it relates to ceramic materials used in insulators of spark plugs.
2. Related Art
Spark plugs, glow plugs, and other such devices used in internal combustion engines are subjected to high temperature environments in the region of about 1,000° C. In general, a spark plug is a device that extends into a combustion chamber of an internal combustion engine and produces a spark to ignite a combustible mixture of air and fuel therein. Specifically, a spark plug typically includes a cylindrical metal shell having external threads that screw into a portion of the engine and further having a hook-shaped ground electrode attached thereto at a firing end of the spark plug. A cylindrical insulator is disposed partially within the metal shell, and extends axially beyond the metal shell toward the firing end and also toward a terminal end. A conductive terminal is disposed within a cylindrical insulator at the terminal end of the spark plug opposite the firing end. At the firing end, a cylindrical center electrode is disposed within the insulator and projects axially out of the insulator toward the ground electrode, whereby a spark plug gap is defined between the electrodes.
In operation, ignition voltage pulses of up to about 40,000 volts are applied through the spark plug to the center electrode, thereby causing a spark to jump the gap between the center and ground electrodes. The spark ignites an air and fuel mixture within the combustion chamber to create high temperature combustion to power the engine. Unfortunately, the high voltage and high temperature environment within the combustion chamber can degrade components of the spark plug. As the spark plug becomes degraded, the intensity of the ignition pulse may become altered, thereby degrading the quality of the spark. Degradation of the spark plug may be caused by dielectric puncture through the insulator which establishes an alternative electric path and consequently the spark may not reliably jump the gap between the center and ground electrodes. The quality of the spark effects the ignition of the mixture of the air and fuel (i.e., the combustion efficiency, combustion temperature, combustion products) thus, the power output, fuel efficiency performance of the engine, and the emissions produced by the combustion of the air and fuel may be adversely affected. Due to an increasing emphasis on regulation of emissions from motor vehicles, the increasing fuel prices, and modern performance demands, it is desirable to maintain a high quality spark for consistent engine performance and emission quality. The longevity of the spark plug, including, quality of the spark, is determined by several factors including the composition of the ceramic insulator material.
The ceramic insulator materials used for the insulator are dielectric materials. Dielectric strength of a material is generally defined as the maximum electric field which can be applied to the material without causing breakdown or electrical puncture thereof. The dielectric strength of spark plugs is generally measured in kilovolts per mil (kV/mil). For a given spark plug design, the insulator dimensions are fixed, thus, dielectric strength is frequently expressed as a breakdown voltage in kV, rather than in kV/mil. A typical value for spark plug dielectric strength for a standard spark plug design used in many applications is on the order of about 40 kV at room temperature. Dielectric strength of the insulators used in spark plugs is also a function of temperature. High temperatures cause an increase in the mobility of certain ions allowing the current to more easily leak through the ceramic. Any leakage of current leads to localized heating which gradually degrades the resistance of the material to dielectric puncture. It has been observed that resistance of insulators to dielectric breakdown tends to decrease over the life of a spark plug due to thermal stress on the spark plug cycling under an applied electric field and due to attendant thermal-electrical fatigue thereof. The exact nature of the microstructural and/or compositional changes are not completely understood, but are believed to be associated with localized heating to temperatures sufficient to bring about partial melting of the ceramic material.
Shunt resistance is another measurable property of ceramics, particularly for those used in spark plugs, and is a measure of the electrical resistance of the material which is generally measured in megaohms. A typical value for spark plug shunt resistance is on the order of about 75 to 125 megaohms at an operating temperature of about 1000 degrees Fahrenheit. Shunt resistance is typically measured on a spark plug as an electrical resistance of the ceramic insulator between the center electrode and metal shell of the spark plug. Therefore, shunt resistance is indicative of the amount of current leakage through the ceramic insulator between the center electrode and metal shell or housing. Whereas dielectric breakdown tends to be a sudden event, shunt resistance tends to be a continuous, parasitic loss of electrical power. Of course, the lower the shunt resistance, the higher the likelihood of catastrophic dielectric failure after the spark plug.
A breakdown in dielectric strength and/or shunt resistance ultimately leads to a spark plug with an electrically parallel path between the center electrode and metal casing in addition to the path across the spark gap between the center electrode and the ground electrode. Shunting of the spark plug is a condition in which an undesirable parallel conductive path is established between the center electrode and the metal casing in addition to the path across the spark gap between the center electrode and the ground electrode. However, in the case of shunting caused by diminished or insufficient shunt resistance, the affect is may simply degrade the spark performance. This additional path even if very small has an adverse effect on the quality of the spark generated by the spark plug. Whereas the parallel electric path is generally due to dielectric breakdown, the effect is generally catastrophic and in many cases significantly reducing or completely eliminating the spark between the center electrode and the ground electrode. A diminished or insufficient shunt resistance degrades the performance of the spark plug and consequently the performance of the engine especially over the service lifetime of the spark plug. As stated above, many times, a degraded shunt resistance will eventually cause a catastrophic failure due to dielectric loss.
As manufacturers continually have increased the complexity and reduced the size of internal combustion engines, spark plugs are needed that have a smaller diameter. Also as manufacturers have continually increased the compression ratio of the engine, requiring higher voltages for the spark to jump the spark gap. Currently, the size the spark plug is limited from further reduction due to the required dielectric strength of the insulator over the service lifetime of the plug, which is directly related to the thickness required for the walls of the insulator. Another factor limiting size reduction is that more manufacturers are demanding a longer service lifetime from spark plugs such as requesting 100,000 mile, 150,000 mile, and 175,000 mile service lifetimes from spark plugs. The longer the desired service lifetime, the higher the required dielectric strength. Also, the higher the required voltage, the higher required dielectric strength. Previously to increase the service lifetime or dielectric strength of a spark plug the walls of the insulator were increased in thickness. However, the current demand for more compact spark plugs for modern engines prevents or limits the use of thicker walled insulators. Therefore, as engines shrink in size and as longer service lifetimes and higher voltages are needed in spark plugs, a spark plug having an insulator with an increased dielectric strength and a reduced wall thickness in size is needed.
Therefore, it would be desirable to produce a spark plug using an improved ceramic insulator material with high shunt resistance that is less susceptible to a breakdown in dielectric strength for extended periods of time at high voltages and high temperatures and, thus, less susceptible to shunting conditions in the spark plug, in order to promote generation of a quality spark and enhanced engine performance.